The present invention relates to a stent for use in a body passageway, comprising a flexible self-expanding braided tubular wall being composed of helically wound wires and having proximal and distal ends. The invention also relates to a method for manufacturing such a stent.
A stent of the type as mentioned in the introduction is described for example in U.S. Pat. No. 4,655,771. The tubular wall is composed of several flexible thread elements each of which extends along a helix with the center line of the tubular wall as a common axis. The thread elements are arranged in two groups of opposite directions of winding crossing each other in a way to form a braided configuration. This is to impart to the tubular body the necessary stability for supporting a vessel. The diameter of the tubular wall can be changed by axial movement of the ends relative to each other. The stent is transluminally inserted into position in its radially compressed state and then subjected to expansion staying in place by a permanent pressure against the inner wall of the body passageway. The stability of the tubular body depends in general from the number of the thread elements, their diameter and material and from the braiding angle of the thread elements at their crossings. It is preferred to have the axially directed braiding angle being obtuse, i.e. larger than 90xc2x0, in order to obtain a large force in radial directions. But the braiding angle also influences the shortening of the stent, which is the reduction of the stent length upon conversion from its compressed to its expanded state. At a given diameter expansion the stent shortens less at braiding angles smaller than around 120xc2x0 than at larger angles.
In the following stents with a braiding angle larger than about 120xc2x0 are referred to as xe2x80x9cnormal-shorteningxe2x80x9d whereas stents having a braiding angle of less than about 120xc2x0 are referred to as xe2x80x9cless-shortening.xe2x80x9d It is an advantage of less-shortening stents that they can be placed more accurately because the practitioner can better estimate the final positions of the stent ends after expansion. The less-shortening feature comes also to fruition when the stent is implanted in a moving hollow organ in which the stent is repeatedly radially compressed, such as in the esophagus, in the trachea or in a pulsating blood vessel. In those cases the reduced shortening of the stent is less traumatic for the inner wall of the hollow organ since the stent ends perform smaller axial movements than normal-shortening stents do. For the aforesaid reasons less-shortening stents are preferably implanted in ostium regions, for example in the aorta next to the entries into the renal arteries or in side branches. Exact placement capability and less axial movement of the stent ends reduce the risk of unwanted perturbation or obstruction of the blood flow by stent ends projecting into the ostium.
However, stents of the less-shortening type comprise smaller hoop strength compared to normal-shortening prostheses due to their smaller braiding angle. A consequence of the lower radial force is a reduction of the self-fixation characteristics with the risk of a local axial displacement of the stent within the body passageway. Moreover, the stent is not stable enough to resist flattening if it is implanted in arched vessels. This means that a more or less strong deformation of the stent cross-section deviating from its original circular shape can partially close the stent.
In EP-A-0 775 471 an improved stent is disclosed comprising a flexible self-expanding braided tubular wall having a proximal segment of smaller diameter and a distal segment of larger diameter and in-between an intermediate segment forming a truncated cone. A covering layer is arranged within the tubular wall. Although the document does not disclose any specific braiding angles the proximal segment will have a similar braiding angle as the above described less-shortening stent and the distal segment will have a larger braiding angle. The different geometry can be derived from the manufacturing methods as described in the document. The large-diameter segment serves as a migration anchor while the less-shortening segment with smaller diameter makes an easier and safer way through curves or at the end of for example a food pipe. But the less-shortening stent segment still has not sufficient shape stability for use in curved areas of body vessels. The cross-section of this segment may be deformed elliptically if bended in curved body vessels as it will occur generally for less-shortening stents. Moreover, because of the conical shape such a stent can be used only at particular areas, such as in food pipes. In addition, it is to be said that the used manufacturing methods are quite expensive.
All documents cited herein, including the foregoing, are incorporated herein by reference in their entireties for all purposes.
It is therefore an object of the present invention to improve a less-shortening stent such that it can be used universally, and more specifically in moving and/or in curved body passageways avoiding migration and flattening deformation thereof. A further object of the invention is to provide a stent which can be manufactured easier.
The term xe2x80x9celevationxe2x80x9d has the meaning of an impression or bulge of the stent wall as well in the negative as in the positive sense, i.e. extending inwardly or outwardly of the tubular stent wall. Accordingly, the tubular wall has at least a local inwardly and/or outwardly formed elevation, whereby the wires are plastically deformed in a way that the number of degrees of freedom for their movement within the braiding is reduced. This means that the mesh cells defined by the braided wires are xe2x80x9cfrozenxe2x80x9d by a reduced capability of the wires to rotate and shift relative to each other at their crossing points. The braided tubular wall remains its less-shortening feature and becomes more stable against radial deformation. A further advantage of the formed elevations is the possibility to make a short stent of the type mentioned in the introduction. Such stents are usually cut from the braiding blank and comprise an unwanted conical shape due to a memory effect from the braiding process. This shape can be converted into a cylindrical tube and conserved by forming elevations on the stent wall.
Where the elevations are distributed regularly over the tubular wall, the stent will be anchored firmly with the tissue of the body vessel without damaging. The homogeneity of the elevation distribution is for example preferred if the stent is to be implanted in a curved area of a body passageway.
More dense distribution of the elevations at the proximal and distal ends of the stent will provide higher stability at these areas for better anchoring thereof with the tissue of the body vessel. This embodiment is preferred if the stent is to be implanted in ostium positions for a safe fixation of the stent ends in order to prevent migration of the stent and disturbing for example the blood flow into a side branch through this ostium. Another preferred application of such a stent is the support of a vessel having a hard plaque stenosis whereby the stent comprises a higher density of elevations in the stenotic region.
In a preferred embodiment of the invention the elevations are formed outwardly so that they can serve as an anchor against stent migration by engaging into the inner vessel wall to be supported. Moreover, the deployment of such a stent with delivery devices as known in the art is enhanced since the retraction of the outer sheath is easier. This results from a reduced friction between the inside of the delivery sheath and the radially outwardly pressing stent touching the sheath only at the elevations.
In another preferred embodiment of the present invention the local elevations have an elongate shape which makes the manufacturing of such stents very easy by using wires to emboss the tubular wall. The elevations may have an arched cross-sectional shape. Preferably the height of the elevations are approximately one to two times the wire diameter of the braid.
These embossments or elevations can be formed helically on the tubular wall, where in a preferred embodiment the helical elevation has a different pitch than the wires of the braid in order to deform as many wires as possible. The elevations may also be formed annularly or in axial direction on the tubular wall depending on the desired effect. Where the elevations are placed annularly the stent wall comprise an improved radial stability, whereas elevations in axial directions impart to the stent a higher longitudinal stability which is especially useful for implantation in the airways.
The manufacturing method according to the present invention is determined by the steps of forming an elongate mandrel having at least one local outwardly bound elevation, forming an elongated tubular braid of spring steel having proximal and distal ends and an inner diameter commensurate with the diameter of the mandrel, engaging said tubular braid over said mandrel, heating the tubular braid on the mandrel, cooling the tubular braid and disengaging the braid from the mandrel. Preferably previous to the disengaging step the braid will be compressed in axial direction.
In sum the present invention relates to a stent for use in a body passageway. A flexible self-expanding braided tubular wall is composed of helically wound wires and has proximal and distal ends, wherein the tubular wall has at least a local inwardly and/or outwardly formed elevation. The local elevations may be distributed regularly over the tubular wall and distributed more densely at the proximal and distal ends. The local elevations of the stent may be formed outwardly and may have an elongated shape. The stent elevations may have an arched cross-sectional shape and/or a height of approximately one to two times of the diameter of the wires. The elevations may be formed helically on the tubular wall. The helical elevation may have a different pitch than the wires of the braid. The elevation may be formed annularly on the tubular wall or formed in axial direction on the tubular wall.
The invention further relates to a method for manufacturing a stent by forming or providing an elongated mandrel having at least one local outwardly bound elevation; forming or providing an elongated tubular braid of spring steel having proximal and distal ends and an inner diameter commensurate with the diameter of the mandrel; engaging the tubular braid over the mandrel; heating the tubular braid over the mandrel; cooling the tubular braid; and disengaging the braid from the mandrel. Prior to disengaging the braid from the mandrel, the braid may be compressed in an axial direction. The steps of heating the tubular braid over the mandrel and cooling the tubular braid may be performed under vacuum condition.